The present invention relates to a flat image display tube for a color television receiver or a computer output display. Several ideas relating to flat cathode ray tubes or image displays are known in television engineering. The applicant has recently applied for a U.S. patent relating to a flat cathode ray tube. The flat cathode ray tube has a small depth relative to an image screen size. Electron beams generated by heating vertically extending linear thermal cathodes are sequentially and vertically switched by a plurality of vertical scanning electrodes extending vertically and arranged perpendicularly to the linear thermal cathodes, transmitted through an electrode having apertures formed therein corresponding to the linear thermal cathodes, horizontally deflected by horizontal deflection electrodes, and directed to a phosphor layer. The electron beams are modulated by applying a modulation pulse voltage together with a heating D.C. voltage to the linear thermal cathodes, or by applying a modulation pulse signal to a modulation electrode arranged close to the electron beam generating electrode.
In the flat cathode ray tube, electron beams emitted from the linear thermal cathodes which are arranged equidistantly in a horizontal direction must impinge the phosphor screen in an equidistant relationship in a horizontal direction when no horizontal deflection voltage is applied. If the equidistance is shifted, the electron beams impinge upon an undesired phosphor which causes a share in hue of the displayed color image. To correct the share in hue, it is necessary to apply a high correction voltage to the horizontal deflection electrodes which requires an expensive and complex circuit.
The share in hue moreover occurs in the above mentioned cathode ray tube when the amplitude of the horizontal deflection is varied.
In addition to those, electron beams impinged onto the phosphor screen through apertures of electrodes are not equal with respect to each other because of the difference of electron emission chracteristics of the linear thermal cathodes, and the difference of the aperture sizes of the electrodes. The inequality of the electron beams make a white image partially dark and deteriorates the quality of the image.